There are a number of recognized methods of depositing layers of silicon dioxide on a substrate. Because of the widespread use of silicon dioxide layers in the electronics industry, the merits and shortcomings of the various deposition methods have been discussed at length in the literature. Overall, it would be of significant advantage for a number of applications to be able to deposit high quality thin layers of silicon dioxide without using high temperatures which can be detrimental to heat-sensitive devices and structures.
One commonly utilized method of depositing layers of silicon dioxide is by thermal oxidation, of silicon, i.e. heating a structure having a layer of silicon or an alloy thereof in the presence of oxygen. While effective, this method requires very high temperatures, on the order of 800.degree. C. or higher, and is therefore unsuitable for applications involving a heat-sensitive substrate.
Silicon dioxide films may also be deposited by chemical vapor deposition (CVD) or low pressure chemical vapor deposition (LPCVD). These techniques likewise require that the substrate surface be heated. Although the temperatures involved are not as high as are required for thermal oxidation, being on the order of 550.degree. C.-600.degree. C., there is still a good possibility that sensitive devices may be adversely affected. In addition, because of defects usually present at formation, frequently it is necessary that silicon dioxide layers deposited in this way be steam annealed at high temperatures to be acceptable for certain applications. The substrates must be transferred to, e.g. an annealing furnace, for this step. This transfer requires additional time and handling and can result in contamination of the surface of the silicon dioxide layer. Silicon dioxide layers may also be formed by sputtering. However, good quality layers can only be obtained by using high substrate temperatures.
There are additional known methods of depositing silicon dioxide layers such as evaporation, particularly by electron beam. While evaporation techniques do utilize lower temperatures, the layers produced thereby generally have strains and/or porosity and are therefore regarded to be inferior to layers produced by high temperature processes such as thermal oxidation, sputtering or steam annealing. The presence of strains is particularly disadvantageous where the layers must be relatively thick. Such layers can be improved by steam annealing which, of course, negates the advantage of low temperature deposition.
In accordance with this invention, there is provided a method whereby layers of silicon dioxide can be deposited at ambient temperature, which layers are equivalent in stoichiometry and low stress to layers formed or annealed at high temperatures.